Instructions
Head to Mahogany Town, then go north to Route 43.
Continue north on Route 43 until you reach Lake of Rage. To get to this route, you can either travel through the tall grass or pay a 1,000P toll at the gatehouse.
Use the Surf ability to go across the lake and engage the Red Gyardos.
Beat the Red Gyardos using attacks it is weak against, such as Thunder. You will receive a Red Scale as a reward.
Travel to Cherrygrove and head north. Speak to Mr. Pokemon to give him the Red Scale in exchange for EXP Share.